1. Field of the Invention
The present invention generally relates to a system for controlling frequency multiplexing modems, and more particularly to a system for controlling frequency multiplexing modems which transfer data via channels controllable independently.
2. Description of the Prior Art
Data communications systems utilizing analog transmission lines employ a modem (modulator/demodulator) for modulating an analog signal and demodulating a modulated signal. Such a modem includes a transmitter unit for modulating a carrier signal with respect to data to be transmitted, and a receiver unit for demodulating a modulated carrier signal received via an analog transmission line so as to obtain the original data.
Recent increase in the scale of communications networks requires a large number of modems to be connected to each other in a point-to-point connection manner.
FIG. 1A is a block diagram of a conventional point-to-point connection network, which includes a center station 7 and a plurality of terminals 8a-8d. The terminals 8a-8d are connected to the center station 7 via modems 1 provided at the side of the center station 7, and via modems 1 provided at the sides of the terminals 8a-8d.
FIG. 1B is a block diagram of another conventional point-to-point connection network, which includes a center station 7A, a repeater station 5 and terminals 8a-8d. The repeater station 5 includes a TDM (Time Division Multiplexing) modem unit 9A, and modems 1 connected to the TDM modem unit 9A and the modems 1 provided at the sides of the terminals 8a-8d. The center station 7A also includes a TDM modem unit 9B connected to the TDM modem unit 9A of the repeater station 5. It will be noted that the modems 1 are respectively provided for the terminals 8a-8d in the same manner as the connection network shown in FIG. 1A.
The connection networks shown in FIGS. 1A and 1B have a disadvantage in that the number of modems and transmission lines increase as the number of terminals increase. This increases the cost for building the networks.
FIG. 2 is a block diagram of an improved network directed to overcoming the above disadvantage. A center station 7B includes an FDM (Frequency Division Multiplexing) modem unit 2. The terminals 8a-8d are connected to the FDM modem unit 2 of the center station 7B via respective FDM modem units 2 provided at the sides of the terminals 8a-8d. A frequency band used for data transmission is segmented into four channels A, B, C and D having respective frequency bands. The terminals 8a, 8b, 8c and 8d are connected to the channels A, B, C and D of the four FDM modem units 2. The center station 7B can independently communicate with the terminals 8a-8d via the assigned channels. That is, four independent communications can take place by means of the single line. Hence, it is possible to greatly reduce the number of modems and the number of cables and to therefore reduce the cost.
However, the improved network shown in FIG. 2 has the following disadvantages. If the quality of reception of signals via one of the four channels deteriorates and communications taking place via the channel having the poor receiving quality substantially become impossible, supply of transmission energy to all of the channels is stopped, and a retraining process for identifying a cause of deterioration of the communications quality and recovering the abnormal channel is carried out for all the channels. Hence, it becomes necessary to interrupt communications via the normal channels.
In the retraining process, it is necessary for all the channels to be pulled in synchronization separately from each other. Hence, it takes a long time to perform the retraining process.